The goal of this research is to analyze and interpret structural changes in neurons and their synapses and to determine how they arise. Differential experience in young animals affects dendritic morphology: more experienced animals have considerably more dendritic material in neurons in many brain regions. The structure of developing synaptic connections is also affected by experience. Differential experience in adults brings about similar dendritic effects, while synaptic effects remain unexplored. Differential environmental complexity brings about larger effects than any other experience manipulation in rats, and these effects are maximal in the occipital cortex. Experiments will use electron microscopic analysis of rat visual cortex to: 1) determine whether dendritic effects of experience reflect differences in the number of synapses in both young and adult animals; these experiments will use stereological techniques, which allow reconstruction of three dimensional tissue characteristics from two dimensional sections; 2) determine in detail the structural effects of experience upon individual synapses and their interrelationships, applying multivariate statistical analyses; this will provide a coherent view of morphological features which may underlie synapse stabilization and efficacy change; 3) assess short term effects of activity state and effects of fixation-induced artifacts on these measures using the in vitro hippocampal slice, with controls for effects of in vitro maintenance upon structure and comparison with in vitro effects on occipital cortex slices; and 4) examine early phases of the synaptic response to differential experience to determine whether synaptogenesis is stimulated by experience or whether differential synapse stabilization accounts for structural effects of differential experience. These experiments address the mechanisms underlying previously-demonstrated structural effects of experience that are hypothesized to mediate developmental information storage and aspects of adult memory. Understanding these processes is essential to development of clinical treatments of learning and memory dysfunction, mental retardation, and other aspects of mental health.